zkvm: support short encoding for scalars and integers

[oleganza]

Problem

Currently const instruction expects a 32-byte canonically-encoded ristretto255 Scalar. This is suboptimal when we want to store small-range parameters such as timestamps (8 bytes) and sequence numbers (<4 bytes).

Proposal

Encode scalars with top all-zero bytes trimmed. To prevent malleability, require that encoding for all scalars.

1. When encoding a scalar in a program: encode the scalar in a 32-byte string, trim all-zero bytes from the end of the string and write it to the program,
2. When reading a scalar, check that top byte is not zero, then pad 32-byte array with zeroes before decoding it and checking if it's canonical.

Under this encoding:

• number 0 is encoded as an empty string,
• 1..255 as [01]..[ff],
• 256..65535 as [00 01]..[ff ff] and so on.

Alternative

We could introduce a separate type "integer", that has fixed-length 8-byte encoding. This is still not optimal for small integers, and introduces an extra type and requires additional rules of conversion to scalars, and a separate instruction to read it.